As conventional propulsion devices for vessels of this type, there are known ones (for example, Japanese Utility Model Publication No. 1-27517) in which the propeller is made like a leaf, a plurality of those propellers are arranged on a propeller shaft, and the propellers have outer tubes provided on outer peripheral parts thereof for giving pressures.
Moreover, there is described in the Specification of Japanese Patent Application Laid-Open Publication No. 5-270486 a device that has a rotatable guide vane provided at a suction inlet of a suction duct, for changing the direction of inflowing water to the suction duct. Further, the Specification of Japanese Utility Model Publication No. 1-29200 has described therein also a device provided with a guide vane for guiding external water to a fin projected under a suction inlet. Furthermore, in Japanese Patent Publication No. 56-40078 for example, there is described also a device in which, when the propulsive force is reduced with drift matters adhering to a grid of a suction inlet, a fork-like member is rearwardly rotated to remove the drift matters.
Still more, there is described in Japanese Patent Application Laid-Open Publication No. 5-278683 also a propulsion device in which a deflection plate is provided at a bottom opening of a steering column, and jet water is frontwardly deflected with a rotatable flap.
However, in conventional water jet propulsion devices, although high-speed travel is permitted, as the blade width of an impeller is short, there is a problem that vortices occur with an increase in number of revolutions of the impeller. Still less, as the travel speed becomes high, the inflow rate of water incoming to a suction duct at a headway direction side decreases, developing low-pressure stream regions, causing cavitation phenomena on blade surfaces of the impeller, so that there may be vibrations and noises and a concurrent roll. And, in a device which has as a measure for prevention of cavitation a mobile guide vane provided at a suction inlet for changing the inflow direction to a suction duct, it may constitute an obstacle to inflowing water in high-speed travel. And, in a device which has a guide blade provided outside a suction inlet, though being able to increase the inflow rate to a suction duct, it may constitute a water fluid resistance, as the vessel moves astern or makes a turn, failing to achieve a rapid directional change.
Further, in a device provided with a grid at a suction inlet, although it has a favorable removability to solid bodies such as a driftwood, those drift matters which have a flexible nature may adhere to the grid or flow into a suction duct, binding an impeller, with a lowered performance causing a reduced propulsive force or a failure to travel. And, in a device in which adhering drift matters are removed from a grid, there is needed a speed reduction of a vessel, in addition to a manual operation. Yet less, in a device which has at a bottom part of a steering column a deflection plate provided for a reverse propulsion guide, it may constitute a water fluid resistance in high-speed travel. The invention of the present application has for its object to provide a propulsion device with improved suction performance and improved travelling performance, and a water jet propulsion device which removes drift matters adhering to a grid, while travelling, and which is small of a reduction in reverse propulsive force, when travelling astern, and free of a sudden braking nor a lateral slip, when performing a change between a forward movement and a backward movement.